Using RC Low-pass Filter to Expand Microprocessor Output Ports

Moore's Law forms an almost inevitable corollary - next year, microprocessors will have more functions, and software teams will come up with more ideas. Unfortunately, however, microprocessor output ports remain the same. It is quite difficult to find a free output port for testing, debugging or standard I/O ports. With the addition of simple hardware, the circuit shown in Figure 1 realizes a single-port "bus" and provides an unlimited number of parallel output ports. 

Figure 1 HC164 peripheral circuit diagram 

The microprocessor output port with RC low-pass filter controls the serial-to-parallel converter HC164. For data input to the serial-to-parallel converter, each bit must consist of a 1-0-1 transition, and the length of the low-level state of this transition data is variable. If the low-level state exceeds the time constant of the low-pass filter, a zero is shifted into the register; if the low-level state is short, a 1 is shifted into the register. In this way, the clock signal and the data signal are combined into one signal. The low-pass filter can separate the clock signal and the data signal (Figure 2). 



Figure 2 The clock signal and the data signal are combined into one signal 



Table 1 whip program output function 

As shown in Table 1, a simple "whip" program implements 8-bit output function. Assuming a low-pass filter time constant of 3μs, the instruction time should be 1μs or less when using a 4MHz or higher frequency crystal. The program uses bit operations on the My_Bit output of the My_Port port. The 

circuit in Figure 1 can control some low-speed devices, such as relays or LCDs, but when the circuit is used for light-emitting diodes, the HC164 will produce annoying flickering when performing write operations. To solve this problem, the circuit in Figure 3 uses another serial input and parallel output register 4096. This chip has a filtered input function, allowing all outputs that have not reached a temporary level to be updated simultaneously. Two monostable circuits provide data and filtered signals. This circuit can be used to control parallel devices, such as a display module based on HD44780. 



Figure 3 Peripheral circuit diagram of serial input/parallel output chip 4096